Previous studies with laboratory animals have shown that, although neither aversive conditioning procedures alone nor salt loading alone increased 24- hr mean blood pressure, a combination of aversive conditioning and high sodium intake were sufficient to engender experimental hypertension within days. The present project extends these findings to behavioral analysis of the regulation of human blood pressure. Two studies have now shown that 14 days of high sodium intake by medical students preparing for examinations increased resting blood pressure (but not heart rate), while neither preparation for examinations in a context of normal sodium intake, nor sodium loading during summer vacation, had any significant effect. Physiological changes accompanying this model of human experimental hypertension have been characterized. Since the development of primary hypertension may involve deficits in assertive behavior, a study has been initiated to assess cardio- respiratory responses of normotensive and borderline hypertensive humans to laboratory situations of simulated social stress. This study also investigates effects of training in relaxation on social stress reactivity and on ambulatory blood pressure. Preliminary data indicate that hypertensive subjects show higher levels of pCO(2) than normotensive subjects under laboratory conditions. This response, if habitual, could mediate increased blood pressure sensitivity to high sodium intake.